Display screen or portion thereof with a graphical user interface



FIG. 1 is a front view of a display screen or portion thereof with a graphical user interface showing our new design.

The outer perimeter, illustrated in the figures by a pair of broken lines, illustrates a display screen or portion thereof and forms no part of the claimed design. The remaining broken lines in the drawings illustrate portions of the graphical user interface that form no part of the claimed design. 

CLAIM We claim the ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 